1. Field of the Invention
The present invention relates to methods and compositions for controlling the gellation rate in an aqueous well fracturing fluid. It particularly relates to a novel liquid complexor used to obtain controlled delayed gellation of borated polysaccharides.
2. Description of the Prior Art
During hydraulic fracturing, a sand laden fluid is injected into a well bore under high pressure. Once the natural reservoir pressures are exceeded, the fracturing fluid initiates a fracture in the formation which generally continues to grow during pumping. The treatment design generally requires the fluid to reach maximum viscosity as it enters the fracture which affects the fracture length and width. This viscosity is normally obtained by the gellation of suitable polymers, such as a suitable polysaccharide. In recent years, gellation has been achieved by cross-linking these polymers with metal ions including aluminum, antimony, zirconium and titanium containing compounds including the so-called organotitanates. See, for instance, U.S. Pat. No. 4,514,309, issued Apr. 30, 1985, and assigned to the assignee of the present invention.
The viscous fracturing fluid being pumped usually encounters high shear in the pipe string during pumping from the surface to the fracture and after entering the fracture, flows at low shear. Recent investigations indicate that the high shear encountered in the pipe string causes extensive degradation of the cross-linked fracturing fluid. Also, high fluid viscosities cause excessive back or friction pressures, limiting the pumping rate, which also affects fracture geometry. These investigations have shown that by delaying the gellation for several minutes during most of the high shear, higher pump rates can be obtained and the fluid generally exhibits better stability. In the case of the metal ion cross-linking systems, the delay in gellation is normally achieved with a delaying additive that binds or chelates the metal ions in solution.
Recently, guar and guar derivaties cross-linked with borate ions have again become popular. In alkaline water having a pH greater than about 7.8, cross-linking of the guar polymer is essentially instantaneous. This action is probably due to the fact that borates easily and readily esterify with 1,2-cissoidial dialcohols or polyhydric alcohols, such as those found on the guar polymer. This esterification is readily reversible, especially at the elevated temperatures found in the well bore, so that free borate ion is always available. As a result, the delay of borate ion cross-linking systems is difficult to achieve.
Certain of the prior art borated guar systems have employed either slow dissolving metal oxides which slowly increase the fluid alkalinity, which in turn promotes crosslinking, or by using calcium borate salts having poor water solubility, relying upon the slow dissolution of borate ions for delay. In both cases, the delay action was based primarily on the slow dissolution of a solid in the aqueous fracturing fluid, resulting in poor control of the delay time and ultimate viscosity of the fluid. U.S. Pat. No. 4,619,776, issued Oct. 28, 1986, to Mondshine, is typical of the prior art in teaching the use of a sparingly soluble borate to achieve some degree of control over the cross-linking reaction.
A need exists for a composition and method for providing more precise control over the cross-linking reaction of a borated aqueous fracturing fluid.
A need also exists for such a composition and method which does not rely upon the slow dissolution of a solid as the basis of the delay mechanism.
A need also exists for such a method and composition which allows selective adjustment of the delay rate at a well site quickly and conveniently.
A need also exists for such a method and composition which provides a reverse reaction to reduce the viscosity of the fracturing fluid with time to facilitate the cleanup of the treatment from the well bore.